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Astron. Astrophys. Suppl. Ser.
Volume 129, Number 2, April_II 1998
Page(s) 267 - 279
DOI: 10.1051/aas:1998184

A&A Supplement series, Vol. 129, April II 1998, 267-279

Received February 27; accepted September 22, 1997

An updated theoretical scenario for globular cluster stars

S. Cassisi tex2html_wrap509, tex2html_wrap511, V. Castellani tex2html_wrap509, tex2html_wrap515, S. Degl'Innocenti tex2html_wrap515, tex2html_wrap519, and A. Weiss tex2html_wrap521

Send offprint request: V. Castellani, Dipartimento di Fisica Università di Pisa, piazza Torricelli 2, 56126 Pisa, Italy,

tex2html_wrap523  Osservatorio Astronomico di Collurania, via Mentore Maggini I-64100 Teramo, Italy
tex2html_wrap525  Dipartimento di Fisica, Universitá de L'Aquila, via Vetoio, 67010 L'Aquila, Italy
tex2html_wrap527  Dipartimento di Fisica dell'Universitá di Pisa, piazza Torricelli 2, I-56126 Pisa, Italy
tex2html_wrap529  Istituto Nazionale di Fisica Nucleare, Sezione di Ferrara, via Paradiso 12, I-44100 Ferrara, Italy
tex2html_wrap531  Max Plank Institut for Astrophysics, Karl Schwarzschild strasse 1, D-85470 Garching b. Munchen, Germany


In the first part of this paper we revisit the history of theoretical predictions for HB luminosities in old Population II stellar clusters, starting from the results of "old" evolutionary computations to introduce in various steps all the available "new" physics. We discuss the influence of physical ingredients on selected evolutionary parameters, finally presenting models which incorporate all the most recent updating of the relevant physics. The evolutionary behavior of such models is extensively investigated for selected choices about the cluster metallicity, discussing theoretical predictions concerning both cluster isochrones and the calibration of the parameter R in terms of the original amount of He in stellar matter. One finds that the "new" physics has a relevant influence on both these parameters, moving cluster ages into a much better agreement with current cosmological evaluations. This scenario is implemented by a further set of stellar models where element diffusion is taken into account. The comparison between theoretical scenarios with or without diffusion is presented and discussed. A discussion of current observational constraints in the light of the updated theory closes the paper.

keywords: stars: evolution; general; fundamental parameters; horizontal-branch

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